1. Field of Invention
The present invention relates to a low-vacuum plasma device for cleaning vacuum chambers and analytical instruments such as Scanning Electron Microscopes (SEM), Scanning Electron Microprobes, Transmission Electron Microscopes (TEM) and other charge particle beam instruments that are subject to contamination problems from hydrocarbons. In particular it is a novel RF plasma apparatus for generating active oxygen radicals from air that can be used for cleaning vacuum chambers such as found in electron microscopes and for other purposes. The oxygen radicals are used to oxidize the hydrocarbons and convert them to easily pumped gases. The apparatus can be added many types of vacuum systems without major modifications and can be used with other gas mixtures to generate a variety of active species. The apparatus is designed to be made from standard vacuum components and electrical feedthroughs.
2. Description Prior Art
The invention described in patent application Ser. No. (09/228,318) a disclosed a general method and apparatus for cleaning an electron microscope using air passed through a low powered RF plasma glow discharge to produce oxygen radicals. Divisional application (09/552,449) of the 09/228,318 application disclosed a cylindrical electrode with multiple apertures that works for generating oxygen radicals from air that operates at low RF power. A number of arrangements can be used to mount this electrode assembly in the vacuum chamber of the SEM that would allow the RF power to be supplied to this cylindrical electrode assembly and to have the reactive gas pass through the glow discharge plasma region. The plasma cleaning action of the cylindrical electrode with apertures of the previous invention was found to be almost independent of how it was mounted in the chamber and how it was enclosed.
It has been well documented that low temperature ( less than 50xc2x0 C.) plasmas of various ionized gases can be used to reactively etch/ash organic materials found on the surface of materials. As xe2x80x9cglow-discharge cleaningxe2x80x9d it has been used by the high-energy physics community to condition the interiors of large vacuum vessels. Named xe2x80x9cplasma etchxe2x80x9d or xe2x80x9cplasma ashingxe2x80x9d, it has been used in the industrial community to clean and etch semiconductor wafers and other bulk materials for many years. In the microscopy community RF or DC plasma, dry-ashing devices are sold by several vendors to clean electron microscope specimens prior to analysis. In this procedure, typically the material is placed in an RF cavity or a DC cavity with a flowing reactive gas. The nature of the gas selected is chosen based upon the desired effect. Argon, nitrogen, air, oxygen or other gas mixtures are commonly used, and gases (BCl3, CF4) may be used to tailor the reaction.
Most of the current literature and recent patents on glow-discharge cleaning and plasma etch is concerned with the use of these processes in semiconductor production. For these processes plasma uniformity, anisotropic etching, and other highly controlled properties are important. The geometry of these systems is very carefully designed for uniform results. A variety of gases can be used for etching and cleaning. Gases such as Hydrogen, Argon, Nitrogen, Oxygen, CF4 and gas mixtures such as air and argon/oxygen have successfully been used for glow-discharge cleaning and plasma etching. Depending on the process the importance of ion sputtering and reactive ion etching varies, but in most of processes the neutral free radicals are the most important reactive species in the plasma. The free radicals, because they are neutral, are able to leave the electric fields of the excitation region and travel throughout the chamber by convection.
For the cleaning and removal of hydrocarbons the reaction with oxygen radicals to produce CO, CO2 and H2O is the most important. These reaction products are quickly removed as gases from the vacuum system. These reactions are the dominant reactions in glow discharge cleaning methods using oxygen as a reactant gas. The glow discharge is used to produce oxygen ions that are then transformed into oxygen radicals by subsequent reactions. The oxygen ions are not needed as the reactive species for hydrocarbons. In the absence of nitrogen ions or other reactive species that destroy O radicals, O radicals are long lived and have the ability to do isotropic cleaning on all surfaces in the chamber. To prevent the formation of Nitrogen ions or other active species that destroy O radical a low temperature plasma is needed. The Ionization potential; of nitrogen is that of oxygen. In low temperature plasmas in low vacuum air the formation of oxygen ions is favored and leads to the formation of oxygen radicals in useful quantities.
Conventional RF or DC plasma cavities for the production of plasmas are usually of four different types: parallel plates for DC and RF capacitivly coupled plasmas, RF inductive coils, RF multiple electrodes, and hollow cathodes. At high vacuums magnets are often used to confine the free electrons to keep the plasma ignited. Parallel plates and inductive coils and their variations are the classical xe2x80x9ctextbookxe2x80x9d designs and well understood. Designs with multiple electrodes that are of opposite RF potential have been devised for filling vacuum cavities with plasmas for etching and cleaning purposes. The hollow cathode design is efficient because it traps the electrons between the walls of the cathode and results in higher free electron densities. Because they are usually purely capacitive or inductive they require high voltages or power to ignite the plasma. Typical peak to peak RF voltages are above 400 V and power is above 100 W. This results in a relatively high energy or high temperature electrons in the plasma. Many of these sources are designed to create plasmas under difficult conditions of either high vacuum or atmospheric pressure where it is harder to ignite or sustain a plasma. At low vacuum between 0.1 Torr and 2 Torr gases become highly conductive and plasmas are easy to ignite and sustain. At these pressures almost any shape of electrode inside a grounded metal vacuum chamber will ignite and sustain a plasma with the application of sufficient RF power if the impedance of the RF circuit is properly matched between the source and the load.
To mount a plasma device on an electron microscope or other vacuum system the designer must consider how to fabricate the device economically. The manufacturing of custom vacuum parts is machine shop is very expensive. Therefore it is useful to use standard vacuum parts for assembly of the system. ISO components (a flange system designed in accordance with ISO, International Standards Organizationxe2x80x94standard 2861/1) are an economical, convenient and simplified means of constructing custom vacuum devices and systems. This flange system consists of two identical, symmetrical flanges, a centering ring that supports an elastomer xe2x80x9cOxe2x80x9d ring and a clamp that compresses the sealing ring between the two flanges. These ISO fittings have a variety of designations from their manufacturers as KF, QF, NW, etc. and are referred to in this specification as ISO KF. An alternate flange system with copper gaskets is suitable for ultra high vacuum applications. This system is known as CF or Conflat(copyright) (a registered trademark of Varian Associates). The CF system is not used on the chambers of electron microscopes but is common in other types of ultra high vacuum instruments. In both the ISO KF and CF flange system there are available RF coaxial feedthroughs and various sized adapters and fittings.
The present invention is directed to an apparatus for housing a cylindrical electrode and supplying RF power and reactive gas to a plasma around the electrode. The apparatus produces oxygen radicals with a reactive gas of air, with the RF power below 15 Watts, and with the vacuum between 0.2 Torr and 1 Torr. At these pressures and power levels sputtering is suppressed which minimizes electrode and chamber damage and sputter contamination. The apparatus is mounted on electron microscopes and other vacuum chambers for the purpose of making oxygen radicals from air for reactively removing hydrocarbons for the walls of the chamber. It is also useful as a device for generating low energy active species from a variety of gases at that uses a low power RF generator and is simple to construct from commercial parts.
Meeting the following mechanical and physical requirements is an object of the present invention:
1. Mountable with simple adapter flange on a port of most commercial electron microscopes.
2 No mechanical interference with other common microscope accessories and detectors.
3 Compact.
4 Simple to make out of commercial parts with standard sized flanges and having little custom fabrication.
5 Have a vacuum gauge and metered gas inlet into the plasma region.
6 Can operate at low RF power and in low vacuum
The present invention covers the use of a small cylindrical chamber, a coaxial electrical feedthrough for RF power, the mounting of the cylindrical electrode with apertures to the feed through, the feeding of an reactive gas such as air into to side of the chamber and the mounting of a pirani gauge to the chamber for measuring the vacuum.